WASHINGTON — Federal authorities are warning law enforcement personnel of a possible terror plot against the New York City subway and train systems during the holiday season, and police are beefing up security in preparation.

An internal memo obtained by The Associated Press says the FBI has received a "plausible but unsubstantiated" report that al-Qaida terrorists in late September may have discussed attacking the subway system.

A person briefed on the matter, speaking to The Associated Press on condition of anonymity because of the sensitivity of the intelligence-gathering work, said the threat may also be directed at the passenger rail lines running through New York, such as Amtrak and the Long Island Rail Road, which are particularly busy with Thanksgiving holiday travelers.

A U.S. counterterror official, also speaking on condition of anonymity because he was not authorized to do so publicly, said senior government officials have been briefed because the FBI very recently received credible information about possible attacks over the holiday season, and authorities are particularly concerned about this long holiday weekend.

FBI spokesman Richard Kolko confirmed only that his agency and the Homeland Security Department issued a bulletin Tuesday night to state and local authorities, and the information is being reviewed.

Department of Homeland Security spokesman Russ Knocke said the warning was issued as a routine matter, but added that there may be an increased police presence in New York and other large cities.

The internal bulletin says al-Qaida terrorists "in late September may have discussed targeting transit systems in and around New York City. These discussions reportedly involved the use of suicide bombers or explosives placed on subway/passenger rail systems," according to the document.

"We have no specific details to confirm that this plot has developed beyond aspirational planning, but we are issuing this warning out of concern that such an attack could possibly be conducted during the forthcoming holiday season," according to the warning dated Tuesday.

NYPD spokesman Paul Browne said they have received an unsubstantiated report and as a result have "deployed additional resources in the mass transit system."

While federal agencies regularly issue all sorts of advisory warnings, the language of this one is particularly blunt.

Intelligence and homeland security officials are working with local authorities to try to corroborate the information "and will continue to investigate every possible lead," the memo says.

Rep. Peter King, the top Republican on the House Homeland Security Committee, said authorities "have very real specifics as to who it is and where the conversation took place and who conducted it."

"It certainly involves suicide bombing attacks on the mass transit system in and around New York and it's plausible, but there's no evidence yet that it's in the process of being carried out," King said.

Knocke, the DHS spokesman, said the warning was issued "out of an abundance of caution going into this holiday season."

No changes are being made to the nation's threat level, or for transit systems at this time, he said.

"However, transit passengers in larger metropolitan areas like New York may see an increased security presence in the coming days," Knocke said.